1. Field of the Invention
This invention relates to ammonia adsorption apparatus to be disposed downstream in the flow of exhaust gas from a denitrator which removes nitrogen oxides present in exhaust gas using ammonia as a reducing agent.
2. Description of the Related Art
FIG. 4 is a flow diagram illustrating a NO.sub.x removal system provided with conventional ammonia adsorption apparatus. In this conventional system, exhaust gas emerging from a gas turbine 1 is conducted through a flue and introduced into a waste heat recovery boiler 2, where it is mixed with ammonia injected by an ammonia injector 3 installed therein. The resulting mixture is then introduced into a denitrator 4 installed downstream, where nitrogen oxides (NO.sub.x) present in the exhaust gas are decomposed into innoxious nitrogen and water. Subsequently, the exhaust gas is introduced into one of two horizontal adsorption towers 5 and 5 disposed in parallel by way of the corresponding exhaust gas duct 7A or 7B, and freed of any residual ammonia by adsorption. Thereafter, the exhaust gas is conducted through an exhaust gas duct 8 and discharged from a stack 6 into the atmosphere. Although each adsorption tower 5 is packed with an ammonia adsorbent, further adsorption cannot be effected after a certain amount of ammonia has been adsorbed. Accordingly, the dampers for adsorption tower 5 are adjusted so that desorption gas comprising a portion of the hot exhaust gas emerging from gas turbine 1 is conducted through a desorption gas duct 9 and introduced into adsorption tower 5 in order to raise the temperature of the ammonia adsorbent and desorb ammonia therefrom. Thereafter, the desorption gas is discharged from stack 6.
After ammonia has been fully desorbed from one adsorption tower 5, this adsorption tower 5 is cooled in preparation for the next adsorption step. This can be accomplished by supplying cold gas to adsorption tower 5 through a cooling gas duct (not shown) and making it flow from the top to the bottom thereof, or by blowing air into the top of adsorption tower 5 by means of a fan (not shown). The above-described steps are repeatedly performed in each adsorption tower. In FIG. 4, while one adsorption tower 5 is engaged in the adsorption step, the other adsorption tower 5 is engaged in the desorption step. Thus, these steps are alternately and repeatedly performed in the two adsorption towers 5 and 5.
Where the adsorption towers are of the horizontal type as seen in the above-described conventional ammonia adsorption apparatus, the desorption of ammonia is effected by using hot exhaust gas for heating purposes in a volume equal to 1/50 of that of the main exhaust gas and passing it through an adsorption tower in the same direction as for the adsorption of ammonia. However, the small volume of gas tends to cause channeling and fails to raise the temperature of the adsorption tower uniformly. Accordingly, the ammonia adsorbed on the adsorbent cannot be fully desorbed within the heating/desorption period, resulting in a decrease in the amount of ammonia adsorbed in the next adsorption step. Moreover, since cooling is effected by making cold gas flow from the top to the bottom of the adsorption tower, the cold gas passes through the adsorption tower locally. Thus, the adsorption tower is not cooled uniformly as a whole and requires a long cooling time. For these reasons, the operation of this apparatus involves various problems including readjustment of the adsorption/desorption time schedule, reconsideration of the number of adsorption towers, and complication of the adsorption/desorption control.